1. Field of the Invention
The present invention relates generally to semiconductor laser devices and more particularly relates to a semiconductor laser device which is suitable for use as a recording and/or readout light source of a recording and/or reproducing apparatus for an optical video disc, a digital audio disc and so on.
2. Description of the Prior Art
Roughly classified, conventional semiconductor laser devices are of a refractive index-guiding type and a gain-guiding type regarding its confinement mechanism in the vertical direction mode.
An example of such refractive index-guiding type semiconductor laser device is shown in, for example, FIG. 1. This semiconductor laser device is formed such that on an N type GaAs substrate 1 are epitaxially grown in turn an N type first cladding layer 2 made of Al.sub.y Ga.sub.1-y As, an N type active layer 3 made of Al.sub.x Ga.sub.1-x As, an N type second cladding layer 4 made of Al.sub.y Ga.sub.1-y As and an N type GaAs capping layer 5 and at the center thereof a high refractive index layer 6 of a stripe pattern is formed by injecting thereinto P type impurity Zn according to the selective diffusion or the like in the direction perpendicular to the sheet of drawing in FIG. 1. The depth of this high refractive index layer 6 is selected substantially deep enough to reach into the active layer 3 or further to the first cladding layer 2 by several thousand .ANG. (angstroms). On the surface of the semiconductor layer 5 is formed an insulated layer 7 made of SiO.sub.2 or the like through which an electrode window is formed. And, one electrode 8 is deposited on the high refractive index layer 6 through the above electrode window in the ohmic contact, while the other electrode 9 is deposited on the lower surface of the N type GaAs substrate 1 in the same ohmic contact. As described above, refractive index differences are generated in the active layer 3 depending on whether there exists the high refractive index layer 6 in the active layer 3 or not and thereby the light emission region is restricted.
On the other hand, an example of the gain-guiding type semiconductor laser device is shown in, for example, FIG. 2. Also in this case, this gain-guiding type semiconductor laser device is formed as follows: On an N type GaAs substrate 1 are epitaxially grown in turn an N type first cladding layer 2 made of Al.sub.y Ga.sub.1-y As, an N or P type active layer 3 made of Al.sub.x Ga.sub.1-x As, a P type second cladding layer 4 made of Al.sub.y Ga.sub.1-y As and an N type GaAs capping layer 5 and at the center thereof a stripe region 10 is formed by selectively diffusing P type impurity, for example, Zn which is extended to the direction perpendicular to the sheet of drawing in FIG. 2. The depth of this stripe region 10 is selected deep enough to reach into the second cladding layer 4. Also in this case, on the semiconductor layer 5 is deposited an insulated layer 7 through which a stripe pattern electrode window is formed. And, one electrode 8 is deposited on the region 10 through the stripe pattern electrode window in ohmic contact, and the other electrode 9 is deposited on the lower surface of the N type GaAs substrate 1 in ohmic contact. In the gain-guiding type semiconductor laser device thus made, the stripe region 10 allows the concentration of a drive current and the drive current injected into the portion just near or under the stripe region 10 allows the laser oscillation within the active layer 3. Namely, the gain distribution due to the concentration distribution in the transverse direction of the carrier injected into the active layer 3 determines the transverse mode.
Although the above refractive index-guiding and gain-guiding type semiconductor laser devices have advantages respectively, they but are not free from respective defects. More particularly, the refractive index-guiding type semiconductor laser device, since its longitudinal mode is a single mode, is poor against noises caused by a returned light when used as the writing and/or readout light source for, for example, the optical video disc and so on. On the other hand, since a so-called beam waist position exists close to the light end of the light emission region, this refractive index-guiding type semiconductor laser device has an advantage that in the practical use the focal position can be determined with ease. Furthermore, since a long distance image in the cross section parallel to the junction plane, namely, a so-called far field pattern is symmetrical with respect to left and right sides, there is then an advantage that a beam spot having less distortion can be obtained with ease as the readout or writing light in the practical use. While, in the above gain-guiding type semiconductor laser, since the beam waist position exists near the inside by approximately 20 .mu.m from the light end of the light emission region and further the far field pattern is asymmetrical with respect to left and right sides, there is a defect that an astigmatism is large and the distortion of the beam spot becomes relatively large. However, in this gain-guiding type semiconductor laser device, its longitudinal mode is multimode so that this device is less affected by the noises caused by the returned light.